Various types of electrical outlet receptacles have been provided for use in marine, commercial and industrial applications with an included interlocking mechanism to reduce the hazards of inserting into or withdrawing from such a receptacle a plug while the receptacle is in a live condition. A common application of this type of outlet receptacle involves refrigerated containers that are transported on land by trucks and on sea by ships. When these vehicles are transported on land, electrical power to drive the refrigeration system is provided by the transporter, such as the tractor pulling the container. When these containers are delivered for loading onto ships, the electrical plugs for powering the refrigeration unit must be plugged into a shore-side power source, and when on board a ship, they must be plugged into a source of electrical power on the ship. When the refrigerated container is at dockside or on board the ship, the source of the electrical power should be sealed water-tight to protect against the wet environment and potential problems of corrosion and rust from salt water and salt air. Additionally, because the current is typically 220 volts or 440 volts with substantial amperage, it is important that the receptacle be provided with apparatus that prevents inserting or withdrawing a plug while the circuit is live, and which apparatus also switches off power to the receptacle when a plug is not present. While various means, most commonly involving o-ring seals, have been widely used to effect waterproof sealing of such units, problems have remained with respect to the interlocking mechanisms for ensuring that a plug cannot be disconnected while the circuit is live and for ensuring that no electrical power is provided to the receptacle outlet when a plug is removed.
Various types of interlocking mechanisms have been used in the past. These have included what is known as the "full interlock" in which a plug must be inserted into the unit, with a switch then being actuated to turn the unit on. An interlock mechanism is provided to prevent the switch from being turned on without a plug in place, and that mechanism also serves to lock the plug in place while the unit is on. The interlock latch must be affirmatively released, cutting the power to the outlet, before the plug can be removed. A problem with this type of structure has occurred when the refrigerated container has been removed, either by a truck or by a crane, without the electrical cable plug of that container having been removed from the power outlet. Because the plug is latched in place, such removal of the container rips the cable either out of the refrigeration unit or out of the plug, causing damage and a dangerously live plug. A second type of interlock is referred to as the "partial interlock" in which an inserted plug is locked in place by a latch whenever the unit is turned on, but which unit may be turned on without a plug inserted. Because the outlet receptacle may be enclosed within a housing in which it is desirable to supply power is also supplied to other outlets within that housing, this arrangement is desirable for certain purposes but remains potentially dangerous in providing a powered receptacle that has no plug in it. A third type of interlock is known as the "spring off" interlock in which a plug must be inserted in order to permit an operating handle to turn the unit on. If a plug is withdrawn without the unit being turned off, a spring will trip the unit to the off, or unpowered, state as the plug is withdrawn. However, if the operating handle is held in the "on" position as the plug is removed, arcing and shock hazard may occur, and that receptacle may be left in a live, powered state, with the attendant danger. Accordingly, it is possible for the "spring off" type of devices to be left in a dangerous, live condition by inadvertent improper use thereof.